The passive skills stack, so if both skills get a defense bonus for example the character will end up with extra defense. The Paladin in specific has lots of good passives to stack.

In the case of the fire mage, his passive fire skills stack, and you get two maelstrom skills. My dual fire mage has a high level maelstrom skill and a level 1 maelstrom skill. I can permanently paralyze enemies by alternating between the two skills. (watch for falling rock though!)

Spells that give you a bonus don't stack, like if you had two of the Paladin's health regen skills it just replaces the skill with the other when you cast it. So in that case, it's mostly pointless.